Such a direct esterification process is known from EP-A-0585071. This process is carried out at a temperature between about 180 and about 240.degree. C. Some time after its start, the esterification reaction substantially ceases. Paraffin wax, which lowers the viscosity of the reaction mixture, is then added to the reaction mixture and esterification resumes. At the end of the reaction the pressure in the reaction vessel is lowered and the excess fatty acid is distilled off. During this distillation a foam is formed.
A disadvantage of the process of EP-A-0585071 is that in the course of the esterification reaction the reaction mass becomes exceedingly viscous, stirring the reaction mixture becomes difficult, and the reaction substantially ceases. The viscosity of the reaction mixture as such is not the problem. It is known in the art to add a viscosity reducing agent, as happens in the above-mentioned EP-A-0585071. In EP-A-0246471 a fluid, semi-solid or solid paraffin is added as a viscosity regulator. Alternatively, an excess of fatty acid is frequently used to keep the reaction mixture stirrable, as in GB 853590, U.S. Pat. No. 2,857,370, U.S. Pat. No. 3,320,292, U.S. Pat. No. 3,394,155, and U.S. Pat. No. 3,383,396.
We have found that the real problem resides in the combination of said viscosity and the formation of a foam, i.e., bubbles of reaction water vapour, which makes mixing of the contents of the reaction vessel difficult, if not impossible. Foaming starts as soon as the reaction product is being formed. As a consequence, the heat transfer is diminished, i.e., the reaction mixture darkens, and water is not distilled off any more, i.e., the reaction substantially ceases. The result is poor conversion and a product with a low active content.
In the process of EP-A-0585071 also during distillation of the excess fatty acid foaming occurs. It has to be monitored carefully by the operator in order for control over the reaction process to be maintained.
In the process of EP-A-0585071 a substantial amount (in examples 1 and 7 about 20 weight percent, based on sodium hydroxyethanesulfonate) of paraffin wax is added. Consequently, paraffin wax is always present in products obtained by this process. For some applications, there is a need for fatty acyl isethionate salts in which no paraffin wax, or for that matter any other consistency (or viscosity) regulator, is present. This for instance is the case with personal care products like shampoos, which need to be clear. It is an object of the present invention to provide a process in which the above-mentioned problems relating to foaming have been solved and which is suitably carried out without the addition of paraffin wax.
Although EP-A-0585071 mentions a molar ratio of the reactants of from about 2.0:1.0 to about 1.0:1.0, in each of the examples an excess of fatty acid is used and this excess is removed by distillation. In this context, we found that foaming became an even greater problem when substantially equimolar amounts of a fatty acid and a hydroxyalkanesulfonic acid salt are used.
In this respect, GB 853590 teaches that it is preferred to use an excess of the fatty acid. Excess fatty acid assists in maintaining the product in a liquid form during the reaction and also helps to prevent or greatly reduce the formation of foam. The same teaching, i.e., a 100% excess or more of fatty acid, is described in U.S. Pat. No. 2,857,370, U.S. Pat. No. 3,320,292, U.S. Pat. No. 3,394,155, and U.S. Pat. No. 3,383,396.
The use of substantially equimolar amounts of reactants is desired for an economical use of starting materials. Further, it is not necessary to distill off excess fatty acid, which results in short reaction times and in a product which has a high active content and is less odourous than the products obtained by the known processes. It is another object of this invention to provide a process in which substantially equimolar amounts of reactants can be used.